It is proposed to study the electrophysiological events that give rise to abnormal rhythms of the heart. The chief technique used is the recording, via intracellular microelectrodes, of the electrical activity of single cells of the heart. The tissues to be studied are the ventricular conducting system (cardiac Purkinje fibers) and the coronary sinus. The Purkinje fibers will be studied with special reference to the recently discovered phenomena of two levels of resting potential in those cells. The effects on those two levels of resting potential of the stimulation of electrogenic sodium extrusion will be studied by varying the degree of sodium loading of the cells and by varying the level of external potassium used to activate the electrogenic extrusion. Inhibition of the sodium extrusion, when needed for control purposes, will be accomplished by using acetylstrophanthidin. Fibers of the coronary sinus will be studied in experiments designed to explain the nature and cause of the depolarizing after-potentials seen in those fibers. Those depolarizing after-potentials give rise to and sustain a form of rhythmic activity called triggered activity and the effect of various drugs (including norepinephrine, acetylcholine and acetylstrophanthidin) and of various ions (including sodium, potassium and calcium) will be studied to see how those drugs and ions affect the after-potentials and thus the triggered arrhythmia.